Hexavalent chromium elimination from wastewater by integrated micro-electrolysis composites synthesized from red mud and rice straw via a facile one-pot method

The widely spread chromium (Cr) contamination is rising environmental concerns, while the reutilization of agro-industrial by-products are also urgently demanded due to their potential risks. In this study, we prepared the integrated micro-electrolysis composites (IMC) through a facile one-pot method with red mud and rice straw. The effects of components relatively mass ratios as well as pyrolysis temperature were analyzed. The XRD, XPS, SEM, FTIR, and various techniques proved the IMC was successfully synthesized, which was also used to analyze the reaction mechanisms. In this study, the dosage of IMC, pH, adsorption time, and temperature of adsorption processes were explored, in the adsorption experiment of Cr(VI), dosage of IMC was 2 g/L (pH 6, 25 °C, and 200 rpm) for isothermal, while the concentration and contact time were also varied. According to the batch experiments, IMC exhibited acceptable removal capacity (190.6 mg/g) on Cr(VI) and the efficiency reached 97.74%. The removal mechanisms of adsorbed Cr(VI) were mainly elaborated as chemical reduction, complexation, co-precipitation, and physical adherence. All these results shed light on the facile preparation and agro-industrial by-products recycled as engineering materials for the heavy metals decontamination in wastewater.


Results and discussion
Physiochemical characterization. The crystalline structure of IMC and other materials was investigated by XRD, as shown in Fig. 1a. After hydrothermal and co-pyrolysis, the distinctive peaks were observed in IMC800-1, with the 2θ of 44.7° and 65.0°, demonstrating the variations of the iron chemical states and existence of Fe 0 (PDF#03-065-4899) 27 . However, these peaks were neglectable in the samples of IMC200-1 and IMC400-1, indicating the pyrolysis temperature acted as essential roles for the formation of Fe 0 species, and lower temperature (< 600 °C) could not form IMC contained Fe 0 . The formation of Fe 0 from biomass and red mud could be explained by the generated hydrogen and CO from biomass could reduce Fe 2+ and Fe 3+ to Fe 0 , and the observed Fe 3 O 4 with the 2θ of 29.8° and 33.4° (PDF#00-034-0417) also proved the transforming processes 28,29 . Meanwhile, the existence of Fe 2 Al 3 Si 3 , Fe 2 SiO 4 , and K 2 FeO 4 , could be attributed to the inherent impurities in the red mud and straw biomass. The influence of different ratios was also analyzed as shown in Fig. 1b. The obvious peaks of Fe 0 were detected in the spectra of IMC800-2 and IMC800-1, and weak crystalline peaks in IMC800-0.5, this phenomenon could be explained by the more biomass in IMC precursors could improve the transformation of iron species, leading to the more complete conversion of Fe. Hence, after this facile one-pot treatment with carbothermal reduction process, the biomass converted into biochar, and iron species turned to Fe 0 , forming the IMC materials, which could contribute the reduction and adsorption of target pollutants 30 .
The chemical states of surface elements in IMC were further investigated by XPS, as depicted in Fig. 1c,d. The revolution of chemical states of C and Fe could elucidate the reactions during IMC formation. The peaks of C 1 s ( Fig. 1c) with the binding energy of 284.78 eV and 284.78 eV related to the structure of C=C and C=O/C-O 31,32 . These results could be contributed to the generation of oxygen-contained functional groups during pyrolysis processes. In the case of Fe 2p peaks, the binding energy with 720.78 eV indicating the successfully preparation of Fe 0 in IMC after pyrolysis (Fig. 1d). The existence of these Fe 0 species (with the molar ratio of 38.37%) was in a good agreement with XRD spectra, which were chemically reactive for the pollutant's elimination. Meanwhile, the presence of Fe 2+ and Fe 3+ was also proved with the binding energy of 711.68 and 725.48 eV for Fe 2p 3/2 , as well as 713.08 eV and 717.28 eV for Fe 2p 1/2 , respectively 33 . These could be explained by the incomplete transformation of iron oxides into chemically reactive states. Hence, according to aforementioned results, we could find that the IMC with reactive species through this facile one-pot hydrothermal and subsequent co-pyrolysis process, these could be expressed as following Eq. (1): The surface morphologies and elemental distribution of different IMC were observed by SEM-EDX (Fig. 2). As shown in results, the irregular pores and unsatisfied porous configurations in IMC800-0.5 were confirmed. After loading with more iron species from red mud, in the SEM results of IMC800-1, the presence Fe 0 was proved with spheres structure, moreover, the wrinkled surface was damaged, and more pore structure occurred 34 . This phenomenon might be attributed to the catalytic properties of iron species during biomass pyrolysis 35 . And these generated pores provided more reactive sites for pollutants. Interestingly, according to the EDS results are exhibited in Fig. 2c-e, there were several minerals in IMC, such as Si, Ca, Al, and so on, these species inherited from red mud, and might could react with contaminants during reaction 28  www.nature.com/scientificreports/ The functional groups in IMC were also detected by FTIR, as shown in Fig. 2f. The wavenumber of 3432, 1632, and 1446 cm −1 was related to the vibration of -OH, C=O, and C-C, respectively 36,37 . These results demonstrated that the abundant functional groups on the IMC800 materials with various red mud loading rates. According to previous works, the peak with the wavelength of 553 cm −1 was attributed to the existence of Fe-O groups, while the peak at 1002 cm −1 ascribed to the stretching of C-O groups 38 . These results offered evidence that loading of iron-derived functional groups, more functional groups formed, despite some groups were occupied by the loading Fe 0 . Therefore, fabrication of Fe0-rich IMC, could also increase the amount of the functional groups, which might play essential roles during pollutants removal 39 .
All these physicochemical characterizations provided information that IMC was successfully prepared, with porous structure and abundant surface functional groups 40 . The optimal preparation conditions were the weight ratio between red mud to rice straw was 1:1 while co-pyrolysis with the temperature of 800 °C. And the Cr(VI) removal capacity was need to evaluate the performance of this IMC material.
Batch adsorption experiments. The Cr(VI) adsorption experiments were conducted to evaluate the removal capacity of various IMC materials. As shown in Fig. 3a, the removal capacity was increasing from 48.02 mg/g of IMC800-0.5 to the 68.03 mg/g of IMC800-2. These results could be explained the more loading of Fe 0 in IMC. According to the former characterization, too much iron species from red mud was detrimental to the formation of species, due to lack of reductive biomass 41 . The removal capacity of BC800 and RM800 was relatively lower (nearly 10 mg/g), this further proved the functionality of IMC adsorbents.
The effects of adsorbents dosage were also investigated as shown in Fig. 3b. With the increasing of IMC800-1 dosage from 10 to 50 mg, the removal efficiency increased from 43.34 to 83.45%, while the capacity decreased from 66.75 to 19.51 mg/g. This could be attributed to the more adsorbents could compete with pollutants and the www.nature.com/scientificreports/ adsorption capacity for per unit adsorbents was inhibited. In order to balance removal efficiency and capacity, the dosage of 40 mg was applied for the further experiments. The influence of solution initial pH during Cr(VI) removal by IMC was also investigated in Fig. 3c. As pH is considered as an essential factor influencing the surface interactions between adsorbent and contaminants, with the increasing of initial solution pH, the capacity of IMC firstly increased during pH 3 to 6, with the removal capacity from 11.05 to 51.66 mg/g. This phenomenon could be attributed to the chemically reduction from Cr(VI) to Cr(III) and followed adsorption on the IMC materials. However, with the continuous pH increment, the adsorption capacity decreased from 35.28 to 8.96 mg/g as the pH value changed from 7 to 9, as the efficiency changed from 70.14 to 14.98%. This could be explained by the existence of Cr(VI) in solution was mainly CrO 4 2− species, which were electrostatic repulsion and competed for reactive sites with abundant OH − species at high pH conditions 42 . In order to further elucidate the surface interactions under various pH, the zeta-potential experiments were also conducted and depicted in Fig. 3d. According to the tests, the zero-point -charge of pH (pH ZPC ) was 6.86, indicating the negative charge when pH at alkaline conditions. These negatively charged surface further supported the analysis of experimental data. In other words higher pH leading to the negative charged IMC surface, which was detrimental to the Cr(VI) removal 42 . Therefore, in the following experiments, the pH 6 was chosen as initial solution pH values.
Adsorption kinetics. The interactive reactions between IMC and Cr(VI) also investigated by adsorption kinetics. As shown in Fig. 4a, the capacity of IMC was increased with the increment of time intervals, this could be attributed to the sufficient surface reactions between adsorbents and adsorbates. The reaction reached equi- www.nature.com/scientificreports/ librium after 720 min with the capacity of 56.3 mg/g. Several models were employed for the kinetics analysis of adsorption processes and the parameters were listed in Table 1. The fitting results indicated the pseudo-firstorder fitted better compared with pseudo-second-order models, with the related coefficients (R 2 ) were 0.999 and 0.991, respectively. But both models fit better, which was possibly the adsorption process is a coexistence of physical and chemical mechanisms 43 . These results demonstrated the existence of chemical bonding between adsorbents and contaminants, such as interactions with polar organic functional groups.

Reaction isotherms. The surface interactions between adsorbates and IMC could be further analyzed by
isotherm study, as shown in Fig. 4b   www.nature.com/scientificreports/ Thermodynamic analysis. With the increasing of temperature, the sorption of Cr(VI) was improved, indicating the endothermic properties of this reactions. Cr(VI) was involved with the removal processes, represented as free energy, enthalpy, and entropy change, labelled as ΔG°, ΔS°, and ΔH°, respectively 45 . According to the results, the Cr(VI) removal was endothermic spontaneous, as listed in Table 1. These results also elucidated the multiple reactive mechanisms between ICM and pollutants, which would be explained in the mechanisms sections.
Removal mechanisms. Firstly, these mechanisms were analyzed by the XRD results, as shown in Fig. 5a.
The complexation peaks with 2θ of 44.7° indicating the iron-containing substances in IMC participated in the adsorption process of Cr(VI), several impurities may also be participating in this procedure, and the corresponding diffraction peaks all have a decreasing trend. the peaks of Fe in IMC800-1 and was substantially weakened. In addition, compounds related to Cr also appeared indicating the formation of crystalline structure. FTIR experiments was also proved the variations of surface functional groups, as shown in Fig. 5b. The weakened -OH vibration at 3432 cm −1 demonstrated that the functional groups were involved in the Cr(VI) removal, which could be reduction or formation of complexations 46 . The same phenomena were observed for the C=C and C-O stretching, indicating the different functional groups contributed to the Cr(VI) removal. with the following Eq. (2) by using hydroxyl groups as an example.
The chemical states of surface elements were comprehensively analyzed by XPS before and after adsorption. As shown in Fig. 6a,b, the C 1 s spectra with the binding energy of 284.52, 285.91, and 288.13 eV, which was associated with the presence of C=C, C=O/C-O, and Cr(CO) 6 , respectively 47 . These results elucidated that Cr element was successfully adhered by IMC. The appearance of Cr 2p in the whole spectra also confirms that Cr was removed by adsorbent, as presented in Fig. 6e,f. Meanwhile, as presented in Fig. 6c,d, the content of Fe(III) increased from 19.18 to 41.83% for IMC800-1, indicating the reactive Fe(II) and Fe 0 species were oxidized during the Cr(VI) removal process. Interestingly, the ratio of Fe(II) almost retained or even increased, but the ratio of Fe 0 decreased. for example, the contents of Fe(II) and Fe 0 were 42.46% and 47.19%,38.37% and 10.98% for before and after adsorption, respectively. This phenomenon demonstrated that with the consumption of Fe(II) and Fe 0 for Cr(VI) reduction, the Fe(III) and Fe 0 could be reacted and consequently generated more Fe(II) species for the reaction 48 . The binding spectra of Cr(VI), with the binding energy ratio of 66.99% and 33.01% with the Cr(III) and Cr(VI), respectively, indicating Cr(VI) could be reduced into Cr(III). All these reaction processes could be explained as following Eqs. (3)(4)(5)(6):  www.nature.com/scientificreports/ As mentioned before, the mechanisms of Cr(VI) removal by IMC (Fig. 7) might be include chemical reduction, physical adherence, formation of complexations, co-precipitation.
We have made a comparison of Cr(VI) adsorption by different modified biochar, as indicated in the following Table 2. www.nature.com/scientificreports/

Conclusions
To sum up, the agro-industrial byproducts red mud and rice straw were applied as environmental benign and inexpensive raw materials for the integrated micro-electrolysis composites synthesis. The facile and time-saving hydrothermal one-pot method followed with pyrolysis processes. Further, the optimal preparation conditions were investigated and obtained, various techniques were employed to characterize the IMC, and IMC materials consisting of Fe0 species were successfully manufactured. Adsorption data of Cr(VI) by IMC were in good agreement with Langmuir isotherm model and pseudo-first-order kinetic model. Based on the thermodynamics results, the interactions fitted well with and, indicating the monolayer was endothermic spontaneous reaction. According to the batch experiments, IMC exhibited acceptable adsorption efficiency of 97.74%. The maximum removal capacity on Cr(VI) immobilization achieved 190.62 mg/g. All these results pave the way for a facile preparation and a new route to develop agro-industrial byproducts recycled functional materials, which is very important in the practical application for the heavy metals decontamination in wastewater.

Materials and methods
Chemicals and raw materials. The potassium dichromate (K 2 CrO 4 ), hydrochloric acid (HCl, 36%), sodium hydroxide (NaOH) were purchased from Tianjin Zhiyuan Chemical Reagent Co. Ltd. All chemical reagents were analytical grade without further modification. Raw rice straw (RS) and red mud (RM) were obtained from Wenshan city, Yunnan province, China. The obtained RS was washed with DI water for several times and then dried at 60 °C in an oven for 24 h. After dried, RS was crushed and sieved through a 100 mesh sieve. The RM was smashed and dried in an oven (60 °C, 24 h) and then passed through the 100 mesh sieve before application. All chemicals were analytical grade without further purification.

Preparation of IMC.
Appropriate amounts of red mud and rice straw were added into the beaker with the mass ratios of 1:0.5, 1:1 and 1:2, respectively. 60 mL deionized water was added and put into the ultrasonic machine for 0.5 h. Then added 40 mL sodium hydroxide solution (0.1 M) and stirred for 2 h at room temperature (25 °C). The materials were put into an autoclave and the temperature was 120 °C for 10 h hydrothermal reaction, this process can closely combine the two substances; After cooling down, the suspension was poured out  www.nature.com/scientificreports/ and dried in an oven with 80 °C for overnight. 15 g of the product was placed in a programmable tube furnace under the protection of N 2 . The heating rate was set as 5 °C/min, and the pyrolysis temperature was increased to 200 °C, 400 °C, 600 °C and 800 °C, respectively, and the holding time was 1 h to prepare zero-valent iron biochar complex. After cooling to room temperature, the samples were washed with deionized water several times until they reached neutral pH (7.0), then dried in a vacuum drying oven at 60 °C. The dried samples were put into sealed bags and marked for preservation.
Characterizations. The crystalize structure of IMC was analyzed by X-ray diffraction instrument (XRD, Ultima IV, Nippon Science Company, Japan). Functional groups were examined by FTIR (Vertex 70, Bruker, Germany) ranging from 400 to 4000 cm −1 via a conventional KBr pellet method. The scanning electron microscope (SEM, Tescan mira4, Czech company) was applied to provide the IMC surface morphology. The chemical states of surface elements (Fe C and Cr) were confirmed by XPS (Thermo-Kalpha, America) analysis, and the C 1 s peaks at 284.8 eV was applied as background to calibrate the results. The surface charge of adsorbents was presented as zeta-potential and tested by the zeta sizer with pH ranging from 3 to 9.
Batch experiments. The concentration of Cr(VI) was detected by UV-Vis coupled with 1,5-diphenyl carbazide method with the testing wavelength of 540 nm. Without special emphasis, in the subsequent adsorption experiment of Cr(VI), the concentration of Cr(VI) solution was 100 mg/L, the dosage of IMC was 20 mg. Each experiment data was sample three times as a parallel experiment, and until the corresponding errors were below 5%, to ensure the homoscedasticity and normality of the experimental data. In order to find a suitable dosage, 10-50 mg IMC were applied and initial Cr(VI) concentration was set at 100 mg/L with 12 h reaction time, the mixed solutions were shaken at a speed of 200 rpm under the temperature of 25 °C. The initial solution varying the pH from 2.0 to 7.0 was adjusted to desired value using 0.1 M NaOH or HCl solution. To analysis the physiochemical interactions between Cr(VI) and IMC, adsorption isotherms were conducted by changing the Cr(VI) initial concentration from 10 to 1000 mg/L, at dosage of adsorbent (40 mg), and the initial solution pH (6.0). The mixture was shaken at 298 K,308 K,318 K for 24 h to ensure the reaction reached equilibrium, respectively. The experimental data of adsorption isotherms were described by the Langmuir model and Freundlich model as listed in the following Eqs. (7) and (8): where q e (mg/g) is the Cr(VI) amount adsorbed equilibrium adsorption capacity, q max (mg/g) is the maximum adsorption capacity, k L and k F are the rate constants of pseudo-first-order model and pseudo-second-order model, respectively. Furthermore, 20 mg adsorbents was dispersed into 10 mL solution contained Cr(VI) with the concentration of 200 mg/L for the kinetics study. Experimental data of kinetics study were fitted by pseudo-first-order model and pseudo-second-order model as listed in the following Eqs. (9) and (10): where q t (mg/g) is the Cr(VI) amount adsorbed on the surface of IMC at time t (min), q e (mg/g) is the Cr(VI) amount adsorbed equilibrium adsorption capacity, k 1 and k 2 are the rate constants of pseudo-first-order model and pseudo-second-order model, respectively.

Data availability
All data generated or analyzed in this study are included in this published article.
(7) q e = k L q max C e 1 + k L C e (8) q e = K F C 1 n e (9) q t = q e 1 − e tk 1 (10) q t = k 2 q 2 e t 1 + k 2 q e t (11) G • = −RT ln K